The present invention is directed to polymeric films having a rheology with controlled and frequently non-linear temperature and shear dependence. More specifically, it is directed to polymeric films having such rheology in the temperature range of 20-150xc2x0 C. It is also directed to polymeric films which are photosensitive.
Polymeric films frequently are used in applications where they are laminated to another film or surface. For storage stability, these films must have relatively high viscosity at room temperature. Without this the films will exhibit creep or flow around the edges, particularly when stored in roll form. However, to laminate effectively, the film must flow and have low viscosity at the lamination temperature, which is typically 70-150xc2x0 C. This is particularly true for photosensitive films which are used as photoresists or solder masks. Such films are laminated onto substrates from which printed circuit boards are manufactured or onto printed circuit boards themselves, and must conform readily to irregular surfaces without air entrapment.
Most polymeric films have a linear viscosity response to temperature, i.e., the log of the viscosity of the film decreases linearly as the temperature increases. It is difficult to design polymeric systems that have a viscosity response that is steep enough to result both in excellent flow under laminating conditions and in excellent cold roll stability.
Furthermore, most polymeric films have a level viscosity response to shear stress, i.e., the log of the viscosity of the film is approximately constant as shear stress increases. It is difficult to obtain films which are stable at ambient temperatures and achieve excellent flow under the high shear laminating conditions.
It therefore would be useful to have a polymeric film in which the viscosity decreased rapidly and non-linearly with increasing temperature. Furthermore, it would be useful to have a film in which the viscosity decreased with increasing shear stress.
The present invention is directed to a polymeric film composition having a rheology wherein the viscosity changes substantially and sharply with temperature and with shear. The film has a viscosity of at least 3xc3x97106 Pa-s at a predetermined first lower temperature and a shear stress of 10,000 Pa; the film has a viscosity of no greater than 1xc3x97104 Pa-s at a predetermined second higher temperature and a shear stress of 50,000 Pa. The lower temperature is generally in the range of 20-50xc2x0 C. and the higher temperature is generally in the range of 70-90xc2x0 C.
In one embodiment, the film composition comprises a comb polymer comprising a backbone and more than one polymeric arm, wherein one of the following conditions is met:
I. (i) the polymeric arms are copolymers derived from monomers comprising 40-80% by weight of a monomer having a functionality capable of hydrogen bonding;
(ii) the number average molecular weight of the polymeric arms is greater than 2500; and
(iii) the weight ratio of the backbone to the arms is less than 3;
OR
II. (i) the polymeric arms a mers derived from monomers comprising greater than 70% by weight of a monomer having a functionality capable of hydrogen bonding;
(ii) the number average molecular weight of the polymeric arms is less than 2500; and
(iii) the weight ratio of the backbone to the arms is less
(iii) the weight ratio of backbone to arm is less than 4.
In another embodiment, the film composition comprises:
A. a comb polymer comprising a backbone and more than one polymeric arm, wherein (i) the polymeric arm is a copolymer comprising 20-40% by weight of a monomer having a functionality capable of hydrogen bonding and (ii) the weight ratio of backbone to arm is less than 3; and
B. a linear polymer having functionality capable of hydrogen bonding.
In another embodiment, the invention relates to a polymeric film composition comprising a linear polymer and/or a comb polymer, wherein the linear and/or comb polymer have functionality capable of hydrogen bonding, and hydrophilic colloidal silica.
In another embodiment, the invention relates to a film composition comprising a first linear polymer having functionality capable of hydrogen bonding, and/or a comb polymer, hydrophilic colloidal silica and a second linear polymer having functionality capable of hydrogen bonding.
The invention also relates to a photosensitive film composition having a rheology wherein the viscosity changes substantially and sharply with temperature and with shear, the photosensitive composition comprising any of the compositions described above and further comprising an ethylenically unsaturated monomer and a photoinitiator system.